ABSTRACT: Chronic conductive hearing loss due to middle-ear diseases can lead to cochlear neuropathy. While our diagnosis and management of middle-ear diseases are not perfect, in part due to incomplete knowledge of the eardrum function for sound transmission. Multiple contradictory theories of eardrum function have been proposed in the past 150 years. These theories are based on incomplete descriptions and misinterpretation of existing eardrum characterizations. Almost all previous measurements of the eardrum response to sound have used steady-state tonal stimuli, while environmental sounds, such as music, noises and speech, are transient in nature. Little is known about the eardrum transient response in the real world. Incomplete knowledge of the eardrum also limits our views on how to repair the diseased ear. Our first goal in this study is to provide evidence to distinguish between these different theories of the eardrum function. Our second goal is to improve our knowledge of the eardrum response to transient sound. Our third goal is to explore clinical application of the measurement of the eardrum transient response for differential diagnosis of middle-ear diseases. To achieve these goals, we propose three aims in this study. Aim 1 employs a state-of-the-art High Speed Holographic Interferometry System (HSHIS) developed in our laboratory to quantify the eardrum transient responses to acoustic and mechanical transient stimuli, at a very high spatial (>100,000 points) and temporal (>100k frames per second) resolution. ?Experimental modal analysis? will be applied to extract three important yet poorly quantified characteristics of the eardrum: (1) natural frequencies; (2) the magnitude and spatial pattern of modal responses; (3) damping. Results in aim 1 will enable us to understand and describe eardrum function with a level of sophistication that has not been available, and resolve contradictory theories of eardrum function. Aim 2 will quantify the eardrum transient response in cadaveric human ears with simulated middle-ear diseases. Results in aim 2 will evaluate eardrum function in pathological ears, and assess the use of our techniques as an objective tool for differential diagnosis of middle-ear diseases. Aim 3 will study the eardrum function in live chinchilla ears in normal and diseased middle ear conditions, such as Otitis Media with Effusion. Very little detailed surface motion of the live eardrum has been reported in the literature, results in aim 3 will fill in that gap. Measurements will be repeated just after euthanasia to directly quantify any post-mortem changes in eardrum function. This aim also serves as a pilot study to evaluate the clinical utility of our system before clinical trials in patients.